36 Organs of the Animal Body : their Forms and Uses. 
Organs of Circulation. — Having given a little time to the 
consideration of the structure and uses of the breathing-organs, 
the observer next turns his attention to the heart, of which he 
has some general knowledge chiefly in regard to the external 
form, size, and colour. The organ, however, as the centre of 
the circulatory system, deserves a little further consideration, if 
only so far as may be necessary to make the inquirer understand 
that the apparently solid mass of muscular or fleshy structure of 
conical form has four distinct cavities, the openings into which 
are guarded by valves perfectly arranged to allow the blood 
to flow in one direction, and to check the attempt to go in any 
other. If the heart and its large vessels can be got out skilfully, 
as a good dissector would do the work, it will appear as in 
Fig. 22 (p. 35) and Fig. 23 (below), which show both sides of 
the organ as it hangs in the chest. 
Fig. 23. — Tlie Heart and principal vessels, riglit face. 
a, right ventrid: ; l>, left ventricle ; c, rigbt auricle ; d, anterior vena cava ; /, pjsterior »ena 
cava ; k, thoracic duct, which carries the lymph poured into it by the absorbents into the lirgs 
anterior veins, where it mixes with the bl'^d going to the rigbt auricle. 
